The term "early music" calls to mind the revival of long-forgotten works that have been gathering dust on library shelves for centuries.

And, indeed, for the Baltimore Consort - who perform at Christ Church Cathedral on Tuesday, presented by Houston Early Music - reviving neglected music is a big part of what they do.

After three decades as a fixture on the early music scene, the Baltimore Consort has 15 CDs to its credit. And many of the works it has played and recorded were unknown until this small band of instrumentalists and singers brought them to light.

But Mary Anne Ballard, a founding member of the ensemble, says the music they play at Christmastime is different.

"A lot of Christmas music has remained in the repertory," says Ballard, who plays stringed instruments (the viol and the rebec) in the consort." So we're talking about an unbroken tradition. Some of these tunes have survived for 400 years."

That means that many of the pieces they'll perform on Tuesday - such as "Greensleeves" and "Here We Come A-Wassailing" - are familiar to modern audiences.

However, people today usually don't know much about the origins of the Christmas songs they know and love. Seemingly straightforward carols can have complicated histories. As Ballard explains, there's often more to them than first meets the ear.

"Sometimes, in our research, we find multiple versions of the same song. So we'll have two versions of "In Dulce Jubilo" on the program. We start with a part-song version from the early 16th century, followed by a lute arrangement from the 17th century."

Some have undergone surprising transformations.

"Around 1900," Ballard continues, "people became interested in taking old melodies and making up new words for them. A song like "Good King Wenceslas" is a good example of this. We have the original tune on our program, called "Tempus Adest Floridum," which is a Scandinavian song about springtime."

Who knew?

Yet this is the kind of detailed research is just one essential part of the Baltimore Consort's elaborate preparations for performances and recordings. Once Ballard has dug up as much information about a given piece, the musicians then have to work it into a playable form.

"We're not like a string quartet," Ballard says, "that can pull a piece by Mozart off the shelf and learn it. We have to make all our own arrangements. And it takes years to really get the music where we want it. When we finally record a piece, it may have been in our repertory for five years."

In January, the Baltimore Consort recorded a Christmas CD (scheduled for release in 2013) that contains a lot of the music they'll play on Tuesday. Yet for this ensemble, a recording isn't so much a finished product as it is a snapshot of an ongoing process.

"Our repertory continues to evolve as we play it," Ballard says. "The better we know it, the freer we are to improvise when we play it."

And, she adds, this is why 30 years of playing Yuletide concerts hasn't blunted the Baltimore Consort's enthusiasm for Christmas music.

"We do it every year, but it's a constantly changing program. Playing Christmas music feels like a natural part of the year's cycle. If we didn't do it, we'd miss it."

Colin Eatock is a writer who covers classical music. He lives in Toronto.